Musnad Ahmad 1153:509

Ishaq ibn Isa narrated to us, he said: Layth narrated to me, he said: Ibn Shihab narrated to me, from Urwah, from Aisha, that Barirah came to Aisha seeking her help with her *kitabah* (contract of manumission), and she had not yet paid anything of her *kitabah*. Aisha said to her: "Return to your masters, and if they wish that I pay off your *kitabah* for you, and that your *wala'* (right of inheritance/patronage) be for me, I will do it." Barirah mentioned that to her masters, but they refused and said: "If she wishes to seek the reward from Allah by paying it for you, let her do so, but let your *wala'* be for us." She mentioned that to the Messenger of Allah (peace and blessings of Allah be upon him), and the Messenger of Allah (peace and blessings of Allah be upon him) said: "Purchase and emancipate, for the *wala'* belongs only to the one who emancipates." She said: Then the Messenger of Allah (peace and blessings of Allah be upon him) stood up and said: "What is the matter with people who stipulate conditions that are not in the Book of Allah, the Mighty and Majestic? Whoever stipulates a condition that is not in the Book of Allah, the Mighty and Majestic, it is not valid for him, even if he stipulates it a hundred times. The condition of Allah, the Mighty and Majestic, is more rightful and more binding."